Plasma spray gun and method for applying coatings on a substrate

ABSTRACT

A method and plasma spraying device for more efficiently depositing heat fusible materials on a substrate. The improved efficiency referred to above is achieved by altering the flow characteristics of a gaseous material as it enters a plasma forming environment such as that produced by a pair of spacedapart arcing electrodes. The flow of gas is controllably altered by a gas distribution ring which is capable of producing a linear or axial gas flow in combination with a helical gas flow. As the mixed flow of gas is converted into a plasma, its speed is accelerated and the axial flow component is gradually converted into a spiraled or helical flow. Whereupon, the heat fusible material introduced into the plasma is thermally liquified and ejected at or near the speed of sound.

OR 3985lsl'40 United States Patent 191- Coucher PLASMA SPRAY GUN ANDMETHOD FOR APPLYING COATINGS ON A SUBSTRATE 1 Nov. 26, 1974 3,676,6387/1972 Stand 219/121 P Primary Examiner-Bruce A. Reynolds Attorney,Agent, or Firm-Trask & Britt [57] ABSTRACT A method and plasma sprayingdevice for more efficiently depositing heat fusible materials on asubstrate. The improved efficiency referred to above is achieved byaltering the flow characteristics of a gaseous material as it enters aplasma forming environment such as that produced by a pair ofspaced-apart arcing electrodes. The flow of gas is controllably alteredby a gas distribution ring which is capable of producing a linear oraxial gas flow in combination with a helical gas flow. As the mixed flowof gas is converted into a plasma, its speed is accelerated and theaxial flow component is gradually converted into a spiraled or helicalflow. whereupon, the heat fusible material introduced into the plasma isthermally liquified and ejected at or near the speed of sound.

7 12 Claims, 5 Drawing Figures 3,851,140. SHEET 20? 2 PATENTELNUVZBIBHg. I l w trode (anode).

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention isdirected generally to plasma guns and particularly to an improved meansand method for introducing a gas into a plasma-forming environment.

2'. State of the Art The use of plasma guns for converting a gaseousmedium through electrical energy into heat and thereby ,achievinga hightemperature, high velocity, gaseous The plasmas high temperature isobtained by applying a voltage sufficient to cause arcing between a pairof spaced-apart electrodes. In so doing, electrons are released from oneof the electrodes (cathode) and as the electrons gain kinetic energyfrom the field, they move at accelerating velocities toward the othervelec- When a free electron field has been developed, the atoms and/ormolecules of the plasma forming material (normally a gas), which hasbeen introduced therein, collide with the free electrons. During thesecollisions some of the kinetic energy of the electrons is transformedand absorbed by the molecules as heat energy. As the temperature of thegas increases, some of the molecules or atoms are ionized yieldingadditional electrons. As ionization continues, the collisions becomemore frequent increasing the conversion of kinetic energy to energies ofheat and ionization. Eventually the gaseous material will take on acharacteristic which is normally referred to as a high temperatureplasma state.

In order for this high temperature plasma state to be sustained, it isnecessary that a continuous source of electrons be provided and that acontinuous supply of plasma-forming material be made available. US. Pat.No. 2,960,594 identifies these prerequisities as (1) minimum powerrequirements (for providing the necessary electrons), and (2) minimumgas flow (for providing the prerequisite number of ions).

This patent further states, in effect, that if the power requirementsand/or if the gas flow falls below these minimums the plasma-formingenvironment will not be sustained and a flash back condition occurs.Flash backing will normally result in a severe drop in temperature andloss of the plasma state.

With plasma guns currently available, relatively high power and gas flowrequirements are required to sustain a plasma-producing environment.This, of course, results in a relatively high cost of operation. As aresult, the use of plasma guns has been limited in commercialoperations. In order for plasma guns to gain broader acceptanc'e and inorder that they may'bcome more economically competitive with otherspraying means, it would be highly desirable if the above-minimumrequirements could be lowered. In so doing, the cost of I operationwould not only be reduced but the life of the 2 electrodes could also beextended thus reducing the down time and expense incurred forreplacement.

SUMMARY OF THE INVENTION A reduction in power and flow requirements foroperating a plasma gun has been achieved by the apparatus and methods ofthis invention which comprise generally a substantially closed chamberwherein a first electrode (anode) defines a nozzle outlet from thechamber and a second electrode (cathode) extends into the chamber inspaced relationship to the first electrode. A means is provided forintroducing an are forming electric current across the electrodes. Aplasma forming gas is introduced into the arc area in a particularmanner such that the kinetic energy of the electrons emanating from theelectrodes is effectively transformed into energies of heat forabsorption by the plasma forming gas. In one embodiment, this isachieved by means of a specially designed gas ring which is capable ofproviding a substantially linear gas flow in combination with a helicalor vortical gas flow. As the gas travels through the nozzle towards theoutlet, the axial gas flow is gradually converted into a helical orvortical gas flow.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional side viewof the plasma spraying device;

FIG. 2 is an exploded cross-sectional side view of the main elements ofthe spraying device;

FIG. 3 is an isometric of the brass housing shown in FIG. 1;

FIG. 4 is an isometric of the insulated housing shown in FIG. 1; and

FIG. 5 is an enlarged partially cut away isometric of the gasdistribution ring wherein the gas flow components emanating therefromare illustrated.

DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in FIGS. 1-4, theplasma spraying device has a handle housing 10 made from an insulatingmaterial such as rubber, plastic, synthetic resin and the like. A hollowelectrical conductive cathode connector 12 runs longitudinally throughthe handle. One end of the cathode connector is adapted with aconnecting means 14 for receiving a water-cooled electrical cable 16.The other end of the cathode connector is adapted with an electricalconductive bushing 18 for receiving and holding the male end of a boredwater exit assembly 20 which in turn is adapted to receive and hold themale end of a bored cathode holder 22. An O-ring 24 is provided toinsure a water-tight connection between the bushing 18 and the waterexit assembly 20. Another 0- ring 26 is provided to serve a similarpurpose between the water exit assembly and the cathode holder 22.

A tungsten or thoriated tungsten hollow cathode 30 has a threaded rearsection for screwing (not shown) into the cathode holder. The front endof the cathode has a conically shaped head 32 which is circumscribed bya gas distribution ring 34 capable of distributing a plasma forming gasinto the area around the cathode in a particular manner. The design ofthe gas distribution ring and its use shall be subsequently described ingreater detail. The water exit assembly 20, the cathode holder 22, thecathode 30 and the gas distribution ring 34 are encompassed and held ina fixed operative posi- Teflon, and the'like. A channel 38, traversingthe insulated housing 36, is adapted at one end with a fitting 40 forreceiving a plasma inlet line 42. The other end of the channel opensinto an annular gas chamber 44 which is in communication with aplurality of slanted channel openings 45 and with at least one otheropening 46 via an intermediate groove cut into the outer wall of the gasdistribution ring. A bored disc plate 48 constructed from a refractorymaterial such as aluminum oxide is mounted to the front face of theinsulated housing 36. The inner circular edge of the bored disc plate 48is in close proximity to the channel openings 45 and 46. The outer edgeof the bored disc is adjacent to a plurality of water carryingpassageways 50 bored axially through the insulated housing 36.

A centrally bored copper anode 54 having a water chamber 56 formed by anannular groove which is in communication with a plurality of axiallybored water carrying passageways 57 is held to the insulated housing 36by an overriding bored anode holder 58. The

copper anode 54 is held in a position such that it is in proximal spacedrelationship to the cathode 30.

The anode holder 58 is provided with a traversing conduit 60. One end ofthe conduit is adapted with a fitting 62 for receiving a water inletline 64. The other end of the traversing conduit is in communicationwith the water chamber 56 of the copper anode 54. The anode holder alsocontains a plurality of bored water carrying passageways 65 fordirecting cooling water toward outlet 16.

Mounted to the front face of the anode holder 58 is a centrally boredbrass housing having a conduit 72 radially bored therein. One end of theconduit is 3 adapted with a fitting 74 for receiving an inlet line 76for feeding a heat fusible material into a flared chamber 77 which is incommunication with the outlet end 78 of the guns nozzle. The outlet end78 is formed by an open ended tubular member 79 having a frustrum shapedflared end piece 83 extending toward the anode holder 58 from a boredend piece 80. The flared end piece 83 is in spaced relationship with amatching flared end section 84 formed around the bore of the brasshousing 70. The space between the two flared pieces 83 and 84 forms theflared chamber 77.

A plurality of axially bored water carrying passageways 86 circumscribethe bore in the brass housing 70. These passageways are in communicationwith an annular cavity 88 of the bored end piece 80. A second set ofaxially bored water carrying passageways 90, of larger diameter,circumscribe passageways 86 and are also in communication with theannular cavity 88. Cooling water enters the annular cavity 88 throughpassageways 86 and leaves the cavity through passageways 90. An O-ring91 insures a seal between the flared chamber 77 and the water carryingpassageways 86 and 88. The bored end piece is held in sealing engagementagainst the brass housing 70 by means of an O- ring 92 and an annularlip 94 extending outwardly from the bored ensi eqr 0- LII LII

A number ofthreaded bolts 96 extend through H I responding number ofmatchedopenings positioned around the peripheral edge of the lip 94, thebrass housing 70, the anode holder 58, the insulated housing 36 andengage a threaded opening in the handle housing 10 for holding theseelements in an aligned and fixed position. Additional O-rings 98 areprovided for insuring a sealing engagement between the above elementsnear the openings through which the bolts 96 pass.

As shown in FIG. 5, the gas ring 34 earlier referred to comprises aceramic sleeve 100 having an annular groove 102 along one of its end. Aplurality of inwardly slanted conduits 104 extend from the groove to theinterior wall 106 of the ceramic sleeve. The conduits are, characterizedin that they all converge (if extended) at a central focal point at ornear the vicinity of the conical head of the cathode. A tangentialopening 108 which is in communication with the annular groove 102 via anintermediate surface groove 109 is positioned anterior to theexit'openings of the slanted conduits for directing a portion of the gasintroduced as a spiraled component about the gas linearly introducedthrough the slanted conduits 104. With this type of gas ring a majorportion of the gas introduced into the area of the cathode possesses alinear component while a minor portion is introduced tangentially toprovide a spiral or vortical component which in effect circumscribes thelinear component.

As the gas passes through the bore (nozzle) 77 of the anode 54 and thebore in the brass housing 70, the linear components of the gas graduallytake on-a spiraling characteristic until reaching the end of the centralbore where the flow is essentially completely helical or vortical innature with little if any of the'linear component remaining.

With the above gas distribution ring for introducing a plasma forminggas into the plasma gun, a substan tially lower minimum gas flow can berealized along with lower power'requirements. This phenomenon isdemonstrated by the following example in which the operating parametersand the test results are reported in Table 1 which immediately followsthe example.

A plasma gun of the type shown in the drawings and hereinabove describedpossessed a nozzle having a diameter of about 0.228 inches and a lengthof about 1.375 inches. Approximately 20 cubic feet per hour of plasmaforming gas was introduced into the gun through the means provided. Thegas introduced comprised a mixture of 8 cfh nitrogen and i2 Cfh argon.An electrical input of amperes and 47 volts was applied to the cathodehaving a diameter of about 0.40 inches.

A heat fusible material comprising primarily tungsten carbide and havinga particle size of about 2550 microns was introduced into the heatedplasma at a rate of about 0.05 lbs/min. As the gas and softened heatfusible material exited from the gun at a velocity approaching the speedof sound, they were directed against a sheet of aluminum. Upon contactthe heat fusible material solidified forming a thin, evenly distributedcoating across the surface of the aluminum sheet.

' OPERATION'OF PLASMA GUN In coating a substrate with a thermallyfusible matestantially linear component.

Generally, the linear component of the gas introduced into the plasmaforming environment will constitute at least of the total volume of gasintroduced.

Preferably the linear component will constitute between about 80 and 90percent of the total gas volume and the helical or vortical componentwill constitute between about 10 and percent of the total gas volume. Asthe gas is converted into a plasma and moves toward the exit end of thenozzle, the linear component will gradually take on a vorticalcomponent. About midway down the nozzle, the vortical componentconstitutes between about and percent of the total gas flow and thelinear component constitutes between about and 70 percent of the totalgas flow.

A direct current is now applied to the spaced-apart electrodes 30 and 54causing an arc to develop between them. The gas passing through theelectrical arc is thermally energized by the electrons released from thecathode transforming the gas into a high temperature plasma. After thelinear component of the hot plasma is gradually converted into asubstantially helical flow, a finely divided thermally fusible materialis introduced into the flow of hot plasma through line 76 and into theflared chamber 77 via intermediate conduit-72. Because of the chambersflared characteristics, the heat fusible material is introducedcountercurrent to the flow of hot plasma. The heat fusible material isthermally liquified as it contacts the hot plasma and is ejected withthe hot plasma gas through the nozzle portion 78 upon a substrate.

PLASMA GUN COOLING SYSTEM During operation of the plasma gun, a flow ofcirculating cooling water is introduced into the gun via line 64 andinto a water chamber 60 of anode 54. The water flows from the waterchamber 60 through a number of water carrying passageways 57 in theanode and through a series of interconnecting water carrying passagewaysand 86 in the anode holder 58 and the brass housing respectively. Thewater finally enters a turnaround water chamber 88 in the guns end piece80. At this point the direction of water flow is reversed and enterswater carrying passageways 90 in the brass housing and eventually intothe hollow cathode 30 via a number of interconnecting water carryingpassageways 65 and 50 carried by the brasshousing 58 and the insulatedhousing 36 respectively. The water cxits through line 16 via watercooled electrical cables 12 located in the handle housing 10.

ELECTRODES The cathode preferably has a conical head as shown in thedrawings; however, a rounded or blunt head can also be advantageouslyused.

Normally, the cathodes conically shaped head will have an included angleof between 45 and 60 and a diameter of between 0.10 inches and 0.125inches. The length of the cathode can be varied or adjusted to provide adistance betweeen the two electrodes which will produce an arcbestsuited for a particular use. In instances where a temperature ofabout 8,000F is desired and where the gun is to be utilized for sprayinga material on a substrate, the distance between the two electrodes willnormally be between about 0.015 inches and 0.100 inches.

The second electrode or the anode nozzle preferably has a length ofbetween 1.125 inches and 1.375 inches and an internal diameter ofbetween 0.200 inches and 0.250 inches. Normally the anode will beconstructed from an electrical conductive material such as copper.

POWER PARAMETERS With electrodes of the type above described and with agas distribution ring of this invention, a direct current of between 30and 200 amperes and a voltage of between 30 and 90 volts are normallyused. The power requirements may vary to a degree depending on the typeand amount of plasma producing gas that is introduced into the electrodearea. For example, a diatomic gas will normally require lower powerrequirements than a monotomic gas. Further, the degree of ionization andthe gas temperatures desired are also factors which must be taken underconsideration in determining the optimum power rq quirements. In mostcases, the most suitable conditions can be imperically determined forthe particular use intended. in all cases, though, substantially lowerpower requirements are required when the gas distribution ring or if thegas flow characteristics wherein described are employed.

PLASMA FORMING GASES To achieve operating parameters wherein a minimumgas flow of around 20 and 30 c.f.h. and an average amperage input ofbetween 50 and amperes are used, a plasma producing gas comprising avolume ratio of between 2 to l and l to l of monotomic gas to diatomicgas is preferred. Excellent results have been obtained wherein a mixturecontaining 60 percent of argon and 40 percent of nitrogen have beenused. As a general rule, argon is more easily ionized than nitrogen atrelatively low energy levels. Mixtures of the above two gases willnormally require an energy level of between those required for theindividual gases. Certain gas combinations also appear to be moresuitable for achieving a particular temperature, especially if thetemperature is below 10,000F. For example, if temperatures of under1,000F are desired, a mixture of gases comprising 60 percent argon and40 percent nitrogen is preferred. However, if a temperature in excess ofl0,000F. is desired, a mixture comprising 50 percent argon and 50percent nitrogen can be used to advantage.

COOLANTS Generally a coolant such as water will be circulated throughthe gun as earlier described. However, other coolants such as glycol,refrigerants, etc, can also be used. In some cases, circulated air orother heat absorbing gases may also be used.

Normally the amount of liquids circulated will vary depending on thedegree of cooling desired. In order to maintain optimum electrode life,it is preferred that the electrodes be maintained at a low temperature.

Generally, as the power requirements are increased, the volume ofcoolant introduced or recirculated is likewise increased assuming thatthe other operating parameters remain relatively constant.

SPRAYING HEAT FUSIBLE MATERIALS Finely divided heat fusible materialsare introduced into the plasma stream and emitted on a substrate in themanner earlier described.

Generally, though, the distance between the nozzle and the substrate isabout 6 to 8 inches when the gun is being operated at an amperage ofbetween 50 and 100 amperes. The distance is normally longer if the heatfusible material has a relatively low melting point and at a shorterdistance if it has a relatively high melting point. The heat fusiblematerial will melt or be softened upon contact with the heated plasmaand will then be accelerated to speeds approaching sonic or supersonicspeeds.

Most all of the synthetic thermoplastic materials such as polyethylene,polypropylene, polyamides, polyvinylchloride, polystyrene orpolytetrafloroethylene are particularly suitable for coating eitheralone or in combination. Other materials such as glass, ceramics,resins, cellulose butyrate, and the like may also be used. The materialto be deposited is iisually introduced as fine particulates having aparticle size of between l25 mesh and 200 mesh.

Any conventional material may be used as a substrate such as the metals,woods, plastics, ceramics, glass and the like.

While the invention has been described with reference to specificembodiments, it should be understood that certain changes may be made byone skilled in the art and would not thereby depart from the spirit andscope of this invention which is limited only by the claims appendedhereto.

I claim:

1. A fluid distribution ring solely for use on an electric plasmaspraying device comprising a substantially circular ring member havingslanted tubular primary openings and at least one secondary openingextending from the outside to the inside surfaces of said ring, saidprimary openings being characterized by their ability to direct a majorportion of a fluid passing through said openings to a focal pointpositioned along-the axis of said ring to provide a substantially linearflow component and said secondary opening being characterized by itsability to direct a minor portion of said fluid tangentially along theinner surface of said ring to provide a substantially helical flowcomponent.

2. The fluid distribution ring of claim 1 wherein said ring contains anannular groove for directing said fluid into said primary openings.

3. The fluid distribution ring of claim 2 wherein said ring contains asecond groove along its outer surface which intersects said annulargroove for directing fluid into said secondary opening.

4. The fluid distribution ring of claim 3 wherein the secondary openingexits from the inner surface of said ring at a point anterior to saidprimary openings.

5. A plasma spraying device comprising a substantially closed chamber, afirst electrode defining a substantially elongated nozzle outlet fromsaid chamber. a second electrode extending into said chamber and inspaced relation to said first electrode, a means for introducing an areforming electric current across said electrodes and'a means forintroducing a plasma forming gas into the area of said are wherein saidmeans includes a gas distribution ring for directing a major portion ofsaid gas along a substantially linear flow path and a minor portion ofsaid gas along a substantially helical flow path which circumscribes thelinear flow path.

6. The plasma spraying device of claim 5 wherein said linear flowcomponent is gradually converted to an increasingly helical flowcomponent as said gas passes beyond said first electrode.

7. The plasma spraying device of claim 5 wherein said gas distributionring comprises a substantially circular ring member having slantedtubular primary openings and at least one secondary opening extendingfrom the outside to the inside surfaces of said ring, said primaryopenings being characterized by their ability to direct a major portionof a fluid passing through said openings to a focal point positionedalong the axis of said ring to provide a substantially linear flowcomponent and said secondary opening being characterized by its abilityto direct a minor portion of said fluid tangentially along the innersurface of said ring to provide a substantially helical flow component.

8. The plasma spraying device of claim 7 wherein said gas distributionring includes an annular groove for directing said fluid into saidprimary openings.

9. The plasma spraying device of claim 8 wherein said gas distributionring includes a second groove along its outer surface which intersectssaid annular groove for directing fluid into said secondary opening.

10. The plasma spraying device of claim 9 wherein said secondary openingexits from the inner surface of said ring at a point anterior to saidprimary openings.

verted into a high temperature plasma.

l l l

1. A fluid distribution ring solely for use on an electric plasmaspraying device comprising a substantially circular ring member havingslanted tubular primary openings and at least one secondary openingextending from the outside to the inside surfaces of said ring, saidprimary openings being characterized by their ability to direct a majorportion of a fluid passing through said openings to a focal pointpositioned along the axis of said ring to provide a substantially linearflow component and saId secondary opening being characterized by itsability to direct a minor portion of said fluid tangentially along theinner surface of said ring to provide a substantially helical flowcomponent.
 2. The fluid distribution ring of claim 1 wherein said ringcontains an annular groove for directing said fluid into said primaryopenings.
 3. The fluid distribution ring of claim 2 wherein said ringcontains a second groove along its outer surface which intersects saidannular groove for directing fluid into said secondary opening.
 4. Thefluid distribution ring of claim 3 wherein the secondary opening exitsfrom the inner surface of said ring at a point anterior to said primaryopenings.
 5. A plasma spraying device comprising a substantially closedchamber, a first electrode defining a substantially elongated nozzleoutlet from said chamber, a second electrode extending into said chamberand in spaced relation to said first electrode, a means for introducingan arc forming electric current across said electrodes and a means forintroducing a plasma forming gas into the area of said arc wherein saidmeans includes a gas distribution ring for directing a major portion ofsaid gas along a substantially linear flow path and a minor portion ofsaid gas along a substantially helical flow path which circumscribes thelinear flow path.
 6. The plasma spraying device of claim 5 wherein saidlinear flow component is gradually converted to an increasingly helicalflow component as said gas passes beyond said first electrode.
 7. Theplasma spraying device of claim 5 wherein said gas distribution ringcomprises a substantially circular ring member having slanted tubularprimary openings and at least one secondary opening extending from theoutside to the inside surfaces of said ring, said primary openings beingcharacterized by their ability to direct a major portion of a fluidpassing through said openings to a focal point positioned along the axisof said ring to provide a substantially linear flow component and saidsecondary opening being characterized by its ability to direct a minorportion of said fluid tangentially along the inner surface of said ringto provide a substantially helical flow component.
 8. The plasmaspraying device of claim 7 wherein said gas distribution ring includesan annular groove for directing said fluid into said primary openings.9. The plasma spraying device of claim 8 wherein said gas distributionring includes a second groove along its outer surface which intersectssaid annular groove for directing fluid into said secondary opening. 10.The plasma spraying device of claim 9 wherein said secondary openingexits from the inner surface of said ring at a point anterior to saidprimary openings.
 11. A method for producing a high temperature plasmain an electric plasma spraying device, comprising introducing a majorportion of a plasma forming gas into an area of a plasma producingelectrical arc in a path which provides a substantially linear flowcomponent and concomitantly introducing a minor portion of said plasmaforming gas into a plasma producing arc along a path which provides asubstantially helical flow component.
 12. The method of claim 11 whereinthe linear flow component is gradually converted into a helical flowcomponent as the plasma forming fluid is being converted into a hightemperature plasma.